The term "waveguide" is used herein to refer to an optical structure having the ability to transmit light in a bound propagation mode along a path parallel to its axis, and to contain the energy within or adjacent to its surface. In many optical applications it is desirable to filter light that is propagating within a waveguide, perhaps an optical fiber, in order to eliminate or redirect light of certain wavelengths or to pass only light of certain wavelengths. Many types of filters, including interference filters, are commonly used for this filtering. However, there are a number of difficulties associated with the use of many types of filters, including interference filters.
First, in some applications the power density of light propagating within a waveguide may be unacceptably high for the filter, having detrimental effects that may include damage to the filter material or reduced filter performance.
Also, filters are typically employed by means of bulky, multiple-optical-element assemblies inserted between waveguides, which produces a variety of detrimental effects. Separate optical elements can be difficult to align in an assembly and it can be difficult to maintain the alignment in operation as well. Each element often must be separately mounted with great precision and the alignment maintained. Also, an increase in the number of pieces in an optical assembly tends to reduce the robustness of the assembly; the components may be jarred out of alignment or may break. In addition, interfaces between optical elements often result in significant signal losses and performance deterioration, especially when an air gap is present in the interfaces. The materials of which the additional elements are composed may also introduce fluorescence or other undesirable optical interference into the assembly.
The size of filtering assemblies is often a problem as well. Not only can it be difficult to manufacture a filter on a small surface area, but also filtering assemblies usually contain bulky light-collimating, alignment and mounting components in addition to the filtering element. However, space is often at a premium in optical assemblies.
In addition, the filtering characteristics of interference filters change depending upon the angle at which light is incident on the filter, and interference filters are generally designed for the filtration of normally incident light. As illustrated in FIG. 1, for many purposes light can be envisioned as numerous light rays 101 simultaneously traveling down the length of a waveguide 102 at different angles. As illustrated in FIG. 1, when an optical filter 103 is placed in the path of light 101 that is propagating within a waveguide 102, much of the light 101 strikes the filter 103 at angles departing significantly from normal to the filter surface 103, adversely affecting filter performance. Therefore, prior to incidence of light upon an interference filter that is normal to the longitudinal axis of a waveguide, it is desirable to reduce the angle between the path of travel of light traveling within the waveguide and the longitudinal axis of the waveguide, referred to herein as the "angular orientation" of the light. Various means have been used for reducing the angular orientation of light traveling within a waveguide for the purpose of interaction with an interference filter, including the construction of elaborate optical assemblies (see U.S. Pat. No. 5,112,127 to Carrabba et al.), the insertion of "ball lenses" or "microlenses" (see U.S. Pat. No. 4,358,851 to Scifres et al.) into the optical path and spot-size enlarging a portion of an optical fiber (see U.S. Pat. No. 4,958,897 to Hisaharu et al.). However, these means for reducing the divergence of light typically involve the addition of multiple, bulky optical elements to an assembly, which introduces a variety of problems as described above.
The angular orientation of light propagating within a waveguide can have detrimental effects in optical assemblies that include filters other than impairment of filter performance. FIG. 2 depicts an optical assembly that includes three optical fibers, one of which has a filter 201 applied to its end face 202. In this optical assembly, light 203 propagating in a first fiber 204 crosses a junction between the optical fibers 205 before impinging upon the filter 201 applied to the end face 202 of the second optical fiber 206. As illustrated in FIG. 2, because of the gap 205 between the first optical fiber 204 and the second optical fiber 206, light having a significant orientation misses the second optical fiber 206 and is lost. Approaches to addressing this problem in the prior art include the use of separate optical elements such as lenses and the fusing of optical fiber end faces to spherical lenses (see U.S. Pat. No. 4,867,520 to Weidel). The same phenomenon can also occur in optical assemblies in which there is no air gap between optical fibers due to materials, such as the filter itself, that are located between waveguide sections and do not provide totally internally reflective surfaces to contain the light.
A long-standing challenge with optical architectures incorporating single mode optical fibers is associated with the small percentage of the fiber end face that is active. Not only is the optically active core extremely small, but a large radial distance separates the optically active core and surrounding region from the optical fiber end face's outer circumference. This characteristic of optical fibers, especially single mode optical fibers, makes alignment of optical fibers extremely difficult when a filter is interposed between them.
Therefore, there is a need in the art for a compact, robust, easily manufactured and high-performance device for filtering light propagating within a waveguide that addresses these difficulties.